Pressure gradient microphone with two diaphragms



g- 1969 PAUL-FRIEDRICH WARNING 3,

v PRESSURE GRADIENT MICROPHONE WITH TWO DIAPHRAGMS Filed Dist. 24, 19662 Sheets-Sheet 1 aknvm FIG.

PAUL-FRlE DRiCH WARNIING 3,459,902 r PRESSURE GRADIENT MICROPHONE WITHTWO DIAPBRAGMS FiledOct. 24. 196

United States Patent US. Cl. 179-121 17 Claims ABSTRACT OF THEDISCLOSURE A pressure gradient microphone having both a high and lowfrequency system. The microphone has two diaphragms facing in oppositedirections. Each diaphragm has its own voice coil and is activatedthrough main and auxiliary sound inlets.

The invention concerns a pressure gradient microphone with twodiaphragms for sound in different frequency ranges.

If a broader frequency range is embraced two microphones can be used:one adjusted to low frequencies, the other to high frequencies. As it iscumbersome to handle two microphones an attempt has been made to houseseveral microphone systems in one casing.

The object of the present invention is to connect both microphonesystems in such a manner that the size of the microphone is kept to aminimum.

Another object of the present invention is to create a single magneticcircuit for two microphones.

A further object of the present invention is to provide the magneticcircuit with a chamber serving as an acoustical RC system to revolveaccoustical phase.

A further object of the present invention is to contrive an RC systemwhich can be regulated.

A further object of the present invention is to fasten the microphonewith springs.

Details of the invention are given in the following descriptionreferring to the drawings.

FIGURE 1 shows a drawing in perspective of a model of the microphone;

FIGURE 2 shows a longitudinal section through the microphone drawn inFIGURE 1;

FIGURE 2a is an enlarged section of the encircled portion of FIGURE 2;

FIGURE 3 shows another longitudinal section through a part of themicrophone perpendicular to the first longitudinal.

FIGURE 1 shows a revolvable casing 3 attached to a stand 1 in which atubular shell 2 is housed on springs. At one end of the casing 3 thereis a sound-permeable removable cap 3a, which may be of wire screen. Theshell 2 and the casing 3 with its sound permeable cap 3a are tubular,i.e. cylindrical, so that the microphone has the appearance of a rod.The microphone has a first main sound inlet opening 5 and a second mainsound inlet opening 6. Furthermore, there are a first plurality ofauxiliary sound inlet openings 7 and a second plurality of auxiliarysound inlet openings 8. The first plurality of auxiliary sound inletopenings 7 is provided with a first plurality of resistances 9, thesecond plurality of auxiliary sound inlet openings 8 with the secondplurality of resistances 4.

The shell 2 has a front shell piece 10 and a back shell piece 11 (seeFIGURE 2). The shell piece 10 has an essentially cylindrical chamber 12,one base of which is the first main sound inlet opening 5 and the otherbase of which is a first diaphragm 13. In the wall of the shell piece 10there are additional openings 14 and 15 which are covered by anacoustical resistance sheet 16. Chamber 12 represents a first main soundchannel on the sound path through the main sound inlet opening 5 and thecovered openings 14 and 15 into this chamber 12 and to the diaphragm 13.

The front shell piece 10 also contains the first plurality of auxiliarysound inlet openings 7. The first plurality of auxiliary sound inletopenings 7 are covered by an acoustical resistance sheet 17. Between theshell piece 10 and the inside of the microphone there are air channels18 which belong to a first auxiliary sound pathway. Starting from one ofthe auxiliary sound inlet openings 7 the first auxiliary sound pathwaycommunicates acoustically with the diaphragm 13. The first main soundpathway passing through chamber 12 reaches the first diaphragm 13 fromone side, the first auxiliary sound pathway the first diaphragm 13 fromthe other side.

This first diaphragm 13 is fixed in its position relative to the firstmain sound pathway and to the first auxiliary sound pathway by the firstfastening means 20.

Between the front piece 10 and the back piece 11 of the shell 2 there isa closed magnetic circuit 21. The closed magnetic circuit 21 has a firstring-shaped magnet 22, a second ring-shaped magnet 23, a firstring-shaped pole piece 24, a second ring-shaped pole piece 25, arodshaped core 26 and a cylindrical connecting piece 27; between thefirst ring-shaped pole piece 24 and the foremost end of the core 26there is a first air slit 28 and between the second ring-shaped polepiece 25 and the hindmost end of the core 26 there is a second air slit29. The position of the core 26 relative to the ring-shaped pole piece24 is ensured by a punched out positioning piece 39, its position withrespect to the ring-shaped pole piece 25 by a positioning piece 40serving as an acoustical absorber. Positioning pieces 39, 40 arenon-magnetic. The individual parts of the magnetic circuit 21 are gluedtogether by a dual-action material so that they constitute a compactunit.

Between the first diaphragm 13 and the first ring-shaped pole piece 24,respectively the front piece of the core 26 there is a chamber 30. Acylindrical chamber 31 is surrounded by the closed magnetic circuit. Athird acoustical resistance 32 is located in this chamber 31 near firstair slit which is prolonged by the holes left in the positioning piece39. Chamber 30 communicates through the first auxiliary sound pathwaywith the outer chamber and through air slit 28 and the third acousticalresistance 32 with chamber 31.

For adjusting resistance 32 accurately there are a nonmagnetic,cylindrical holder 33 drawn over the core 26 and onto which resistance32 is pushed, and a mechanical force distribution means. These pressresistance 32 between them and a collar 33a of the holder 33. The collar33a has round holes through which air slit 28 is prolonged up toacoustical resistance 32. The mechanical force distribution means has ahollow rod 34 with a flange 34a and an eccentric. The eccentriccomprises a cam 36, a screw 37 and a nut 38. The core 26, the connectingpiece 27 and the cam 36 have bored holes. The screw 37 fits into theseholes. Before the ring-shaped magnet 23 is assembled, the cam 36 isfastened to the shaft of the screw 37 by means of a worm screw 36a.After the closed magnetic circuit 21 has been assembled, the screw 37together with the cam 36 can be turned and made to exert a rather slightpressure on resistance 32, thereby regulating the acoustical resistance.The desired position can be held by tightening the nut (friction betweencam 36 and holder 33 as well as between connecting piece 27 and nut 38prevents undesired slipping). Sheets 35 are pasted on to the hole inconnecting piece 27 and act as sound absorbers. As shown in FIGURE 3 apassage 41 is formed by spaces left in ring-shaped magnet 22, connectingpiece 27, ringshaped magnet 23 and positioning piece 40. Passage 41 isacoustically insulated from chamber 31 by positioning piece 40 andclosed off from the outside by a plate 42 so that the second main soundinlet opening 6 remains free. The second main sound inlet opening 6,passage 41 and the second air slit 29 form a second main sound pathway.

'Second fastening means 44 hold the second diaphragm 43 in positionrelative to the second air slit 29 in the same manner as the firstfastening means 20" hold the first diaphragm 13 in position relative toair slit 28. There is a chamber 45 between the second diaphragm andringshaped pole piece 25, respectively core 26. The mass of thediaphragm and of the attached coil and the coupled acoustical masses aswell as the reduced tension of the diaphragm allow the second diaphragmto accommodate lower frequencies than the first diaphragm.

The back piece 11 of the shell 2 comprises a pipe piece 46, an anteriorpartition 47, a posterior partition 48 and a thin pipe 49. The secondplurality of auxiliary sound inlet openings 8 is located in theposterior partition 48. These are covered by a resistance sheet 50. Thesecond acoustical resistance 4 is formed in this manner. The thin pipe49 is fastened near an opening 51 of the anterior partition in such amanner that a sound passage is formed between the auxiliary sound inletopenings 8, the thin pipe 49 and opening 51. This sound passage is thesecond auxiliary sound pathway. The dimensions of the thin pipe 49 aresuch that the mass moved through the second auxiliary sound pathwaycorresponds approximately to the mass moved through the second mainsound pathway. The anterior partition 47 has an additional opening 52with a corresponding fourth acoustical resistance 53. There is a chamber54 between the anterior partition 47 and the second diaphragm 43 and achamber 55 between the anterior partition -47 and the posteriorpartition 48.

Chamber 54 communicates on the one hand through the auxiliary soundpathway with chamber 55 and through the auxiliary sound inlet openings 8with the outer chamber and, on the other hand, through opening 52 andthe fourth acoustical resistance 53 with chamber 55.

The casing 3 fastened to the posterior partition 48 and thus the back ofthe microphone by means of a soundpermeable spider spring 56 or anothersound-permeable connecting piece and a screw 57. The casing 3 has asliding ring 58 so that the microphone, which lies inside, can movefreely with respect to the casing 3. The sliding ring 58 is located onthe middle part of the microphone but so far back that the secondplurality of main sound inlet openings 6 are not covered by it.

Near the first air slit 28 the first diaphragm 13 has a first voice coil60, near the second air slit 29 the second diaphragm 43 has a secondvoice coil 61. The electrical potentials of these voice coils areconducted by double electrical leads 63 and 64 to filters 65 and 66.Filters 65 and 66 are connected by a lead 67 and terminals 68 over leads69 and 70. One of the circuit clamps is connected by another lead 71with shell 46 and thereby with the mass. The double leads '63 and 64pass out of chamber 31 through an acoustical absorber 72 and intochamber 55 though another acoustical absorber 73. Leads 69 to 71 passout of chamber '55 through another acoustical absorber 74. Theacoustical absorbers 72, 73, 74 are made of synthetic glue which closesthe openings through which the leads pass.

In practice the sound pathway is 22 mm. wide and 18 mm. long in order toadjust diaphragm 13' to a relatively high frequency range. The diameterof openings 14 is 8 mm., of openings --4- mm. Openings 14 are located 14mm. from the first diaphragm 13 and openings 15 7.5 mm. from diaphragm13. The distance of the first auxiliary sound inlet openings 7 fromdiaphragm 13 is 9 mm. (The distances indicated are reckoned to the planeof the diaphragm 13, which is given by fastening means 20.) The form ofthe main sound pathway can deviate .4 from the suggested dimensions.Independently thereof the distance from the first auxiliary sound inletopenings 7 to the first diaphragm 13 should not exceed 20 mm. and thedistance from the second auxiliary sound inlet openings 8 to the seconddiaphragm 43 should not be less than 50 mm. In this manner diaphragm 13is adjusted to relatively high frequencies and diaphragm 43 torelatively low frequencies. The first auxiliary sound pathway need notbe constituted by several first auxiliary sound inlet openings 7; asingle opening is sufficient. The second main sound pathway need not beconstituted by a single opening 6 and corresponding sound passage 41; itmay consist of several parallel openings and sound passages. The secondauxiliary sound pathway need not be formed as indicated by severalopenings *8, a chamber 55 and a mass passage (inside of pipe 49); thesecond auxiliary sound pathway may also be otherwise constructed. Inparticular, the second auxiliary sound pathway may be identical to thefirst auxiliary sound pathway. This means that chamber 55 may beeliminated from the second auxiliary sound pathway led through theacoustical resistance or resistances 4, sound passages, as formed bypipe 49, directly into chamber 54.

In the described model the closed magnetic circuit is glued together andto the front and back shell parts 10 and 11. It is also possible to jointhe front and back shell parts 10 and 11 so that the closed magneticcircuit lying between them is held together mechanically. It is alsopossible to use a one-piece shell to hold the closed magnetic circuittogether mechanically.

It is further possible, inlieu of a single magnetic circuit, to use twoseparate magnetic circuits which touch on their diaphragm-free sides.Whereas the anterior diaphragm faces the direction in which the soundtravels, the posterior diaphragm is turned by and faces away from thedirection of the sound. With this arrangement it is possible toeliminate the base plate of the magnetic circuit whereby every magneticcircuit presents an opening on the side facing the diaphragm. Theseopenings are joined together, forming a corresponding chamber 31. Thischamber can be divided by a sound absorber (similar to positioning piece40 in FIGURE 2). Each sub-chamber created in this manner can beacoustically related to the neighboring diaphragm. This acousticalrelating occurs through sound inlet means such as air slits similar toair slits 28 and 29. Moreover, a chamber corresponding to chamber 55 canbe provided by the shell for the posterior microphone; the variations ofthe model shown in FIG- URE 2 can be made.

I claim:

1. A pressure gradient microphone comprising the following items:

a closed magnetic circuit; this closed magnetic circuit has at least onemagnet and a first and second air slit; a first diaphragm and firstfastening means for this diaphragm; the first fastening means hold thediaphragm in position relative to the first air slit of the closedmagnetic circuit;

a second diaphragm and second fastening means; the

second fastening means hold the diaphragm in position relative to thesecond air slit of the closed magnetic circuit;

a second voice coil; this second voice coil is attached to thle seconddiaphragm and extends into the second an s it;

a first main sound pathway; this item has at least one first main soundinlet opening and communicates with the first diaphragm;

a first auxiliary sound pathway; this item has at least one firstauxiliary sound inlet opening and communicates with the first diaphragm;

the first main sound pathway reaches the first diaphragm from one sidewhile the first auxiliary sound pathway reaches the first diaphragm fromthe other side;

a second main sound pathway; this item has at least one second mainsound inlet opening and communicates with the second diaphragm;

a second auxiliary sound pathway; this item has at least one secondauxiliary sound inlet opening and communicate-s with the seconddiaphragm;

the second main sound pathway reaches the second diaphragm from one sidewhile the second auxiliary sound pathway reaches the second diaphragmfrom the other side;

electrical connection means for the first and second voice coil;

a shell; the shell surrounds the closed magnetic circuit; the first andsecond diaphragm, the first and second fastening means, the first andsecond voice coil, the first and second main sound pathway and at leastthe first and second auxiliary sound pathway;

whereby the closed magnetic circuit constitutes a single unit.

2. Microphone comprising the items of claim 1 and ineluding a firstacoustical resistance; this first acoustical resistance is related tothe first auxiliary sound pathway; a second acoustical resistance; thisitem is related to the second auxiliary sound pathway;

whereby the first and second auxiliary sound pathway constitutes achannel;

whereby the first auxiliary sound pathway is shorter than the secondauxiliary sound pathway.

3. Microphone comprising the items of claim 1 and a sound absorber;

whereby the closed magnetic circuit contains an inner first chambercommunicating with the first auxiliary sound pathway and separated fromthe second air slit by the second absorber;

whereby the shell has an inner second chamber communicating with thesecond auxiliary sound pathy;

4. Microphone comprising the items of claim 3 and a second chamberrelated to the second diaphragm as well as third and fourth acousticalresistances;

whereby the third acoustical resistances separate the first auxiliarysound pathway from the first chamber; the sound absorber separates thefirst chamber from the second main sound pathway and the fourthacoustical resistance separates the second space from the secondchamber.

5. Microphone comprising the items of claim 1, whereby the first mainsound inlet opening, the shell and the first diaphragm delimit the firstmain sound pathway and the shell has openings leading to the main soundpathway.

6. Microphone comprising the items of claim 1, whereby the closedmagnetic circuit has at least one air channel constituting the secondmain sound pathway.

7. Microphone comprising the items of claim 1, whereby the air masses ofthe second main sound pathway and the second auxiliary sound pathway areabout equal.

8. Microphone comprising the items of claim 1, whereby the distancebetween the first auxiliary sound inlet opening and the first diaphragmis not greater than 20 mm.

9. Microphone comprising the items of claim 1, whereby the distancebetween the second auxiliary sound inlet opening and the seconddiaphragm is not less than 5 cm.

10. Microphone comprising the items of claim 5, whereby the first mainsound pathway is a maximum of 20 mm. in length.

11. Microphone comprising the items of claim 4 with a mechanical forcedistribution means; this mechanical force distribution means includes arod located inside the first space and an eccentric which articulateswith one end of the rod by means of an adjustable cam whereas the otherend of the rod articulates with the third acoustical resistance.

12. Microphone comprising the items of claim 1 as well as a casing and asound porous, spring-like connecting piece, whereby the casing surroundsat least the middle and posterior portion of the microphone and thespringlike connecting piece is fastened to the casing and to theposterior portion of the microphone.

13. Microphone comprising the items of claim 12, whereby a sliding-ringbearing rides between the casing and the middle portion of themicrophone.

14. Microphone com-prising the items of claim 12 as well as a soundporous, removable cap for the tip of the casmg.

15. A pressure gradient microphone containing a first and secondmicrophone system; the first microphone is adjusted to a higherfrequency range than the second microphone; the first microphone systemhas a first diaphragm facing the direction of the sound; the secondmicrophone system has a second diaphragm facing away from the directionof the sound i.e. turned at an angle of main and auxiliary soundpathways are related to the first dia phragm which they adapt to a highfrequency range; main and auxiliary sound pathways are related to thesecond diaphragm which they adapt to a low frequency range; the firstand second diaphragm have voice coils; these first and second voicecoils are connected to the terminals by electrical connecting means.

16. Pressure gradient microphone according to claim 15, whereby thefirst closed magnetic circuit has an opening on the side facing awayfrom the first diaphragm and the second closed magnetic circuit has asecond opening on the side facing away from the second diaphragm; thefirst opening and the second opening are joined; the space thus formedhas sound inlet means.

17. Pressure gradient microphone according to claim 16, whereby theinner space formed by the first closed magnetic circuit and the secondclosed magnetic circuit is partitioned by an acoustical insulator, sothat part of the space communicates acoustically with the firstdiaphragm and the other sub-space communicates acoustically with thesecond diaphragm.

No references cited.

KATHLEEN F. CLAFFY, Primary Examiner ARTHUR A. MCGILL, AssistantExaminer US. Cl. X.R.

